Internal combustion engines have long been used as power sources in a broad range of applications. For instance, since the development of electrical power and its widespread availability liquid fuel internal combustion engines have been widely used to power electrical generators. As with the burning of most fuels, however, certain gaseous and particulate pollutants can be problematic byproducts. In more recent years, gaseous fuel internal combustion engines have become commonplace, particularly in electrical power production operations. While many gaseous fuels inherently burn cleaner than liquid fuels such as gasoline and diesel, gaseous-fueled engines are not without emissions control problems. Moreover, increasingly stringent government regulations create a continuing challenge for engineers in designing and operating internal combustion engines that meet or exceed present and future standards for clean and efficient operation.
It is widely understood in the art that minimizing changes in engine speed can facilitate cleaner burning of the fuel, as well as improving operating efficiency. Moreover, many engines will have a particular speed or engine speed range associated with a desired level of performance. To this end, many engines are operated at or close to a predetermined engine speed set point. Where an internal combustion engine is coupled with an electrical generator, however, it has proven difficult to avoid unduly changing the engine's speed when a change in the electrical load on the generator occurs, for example, with relatively large increases or decreases in power demand on an associated electrical grid.
In certain engine designs an electronic controller adjusts the engine throttle upon detecting a load change or anticipated load change. Such designs often attempt to correlate throttle position with engine load and, in theory, will eventually adjust the intake manifold pressure to a point where the engine output torque suits the adjusted load. In many such designs, however, engine speed can fluctuate undesirably before settling, if at all, to within an acceptable range of the set point, due at least in part to rapidly changing intake manifold dynamics.
In particular, relatively rapid changes in the temperature and pressure of the gases in the intake manifold may briefly have a greater impact on engine torque output than changing throttle position. Such problems can be particularly acute where a turbocharger is coupled with the engine, imparting added complexity to the intake manifold dynamics. Where the engine torque output is not appropriate for the load, the imbalance may cause the engine to speed up or slow down, rather than settling toward a constant speed. Where intake manifold pressure and/or temperature are regularly or continuously fluctuating, the density of the gas mixture entering the engine cylinders may be changing, affecting the engine torque output and making it quite difficult to maintain the engine speed within a desirable range by adjusting throttle position. Adjustments in the throttle may be eclipsed by intake manifold dynamics.
Compounding the challenges presented by intake manifold dynamics, the relationship between throttle position and load tends to be difficult to predict. Engine speed and load, turbocharging, throttle position and intake manifold pressure and temperature may be cross-coupled variables, wherein a change in only one of the values may affect the others. In certain engine systems, for example, adjustment of throttle position or throttle angle will not necessarily relate linearly with the engine's output torque. In other words, increasing or decreasing throttle angle will not necessarily result in a corresponding increase or decrease in engine output torque. In some instances, an increase in throttle may be inversely proportional, at least temporarily, with engine output torque, particularly at higher engine speeds and loads and in highly turbocharged engines. Thus, in earlier designs an attempt to compensate for an increased load demand, for example, by opening the throttle could actually momentarily decrease the engine output torque, compounding speed control problems.
One example of an internal combustion system directed to intake manifold pressure control is described in U.S. Pat. No. 6,715,476 to Gopp et al. In Gopp et al, an engine is described having an exhaust gas recirculation system connecting with an intake manifold. An exhaust gas recirculation valve is adjusted to control intake manifold pressure, adjusting intake manifold pressure toward a desired pressure that is based on the position of an automatically controllable airflow actuator. The Gopp et al. design appears to have certain applications, for example controlling emissions, but does not appear well suited for controlling engine speed through adjustment of intake manifold pressure, as the disclosure is primarily concerned with increasing or decreasing the level of inert exhaust gas in the intake manifold.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.